The object of the present invention is vanadium dioxide microparticles, a method for preparing said microparticles and their applications, notably for surface coatings in which they are incorporated.
In a first aspect, the invention relates to microparticles of vanadium dioxide of formula V1xe2x88x92xMxO2 in which 0xe2x89xa6xxe2x89xa60.05 and M is a doping metal, said microparticles having a particle size of less than 10 xcexcm, notably less than 5 xcexcm, preferably in the order of 0.1 to 0.5 xcexcm.
The doping metal may be selected from transition elements which offer an ionic beam greater than that of vanadium such as for example Nb or Ta or an electronic contribution such as for example Mo or W, W and Mo being preferred.
In a preferred aspect, the microparticles according to the invention are constituted of doped vanadium dioxide of formula V1xe2x88x92xWxO2 in which x is between 0 and 0.02.
The vanadium dioxide microparticles according to the invention may notably be used in the technical sector of coating compositions intended to be essentially deposited in thin layers in the form of a film or a leaf, such as paints, varnishes and any other type of coating that may be deposited in successive layers.
The aim of the invention is therefore to use the vanadium dioxide microparticles described above for carrying out an  less than  less than intelligent greater than  greater than  material which automatically reduces the transmission of solar rays in the domain of infra-red rays, when the material reaches a given temperature level. It is thus possible to benefit from the energy of the infra-red rays below the fixed temperature and to eliminate the excessive heating above this temperature.
One of the principal applications of the vanadium dioxide microparticles according to the invention is their use in coatings intended to be affixed on the facades of buildings exposed to bad weather. The dark coloured coatings exposed to the sun""s rays heat up much more than those of light colour. They therefore undergo expansion-contraction cycles of very high amplitude which cause a premature degradation of the coating sheet. It is therefore not possible at the present time to guarantee a dark paint whose luminous luminance is lower than 35%.
This phenomenon may be limited by the addition of a vanadium dioxide pigment to the paint whose fixed transition temperature should be in the order of 25xc2x0 C. for example.
Another application is that of the protection of transparent or translucent surfaces which must allow visible rays to pass through them, such as in greenhouses, verandas, housing glazings, but whose internal temperature needs to be controllable, such a use may also be envisaged within the context of glazings and coachwork of cars and all other transport vehicles.
In summer, by reducing the entry of incident solar energy into buildings, the coating enables reducing the needs for air-conditioning and, on the other hand, in winter, the coating limits the dissipation of heat towards the exterior. Thus the coating advantageously allows an economy in energy.
One of the objects of the present invention is in fact the controllability of the transfer and the absorption of calorific energy at the surface of a wall without necessitating specifically transforming or treating the material thereof, but by depositing a coating following any known method, such as is practised with paints, it being possible for said coating according to the invention to be itself such a paint, enabling an economic implementation and manufacture.
Now, various ionic or molecular compounds are known which, under the effect of a variation of temperature, can change the optical properties, principally the colour, linked to a change of electronic structure: such compounds are called  less than  less than thermochromic greater than  greater than  compounds. By extension, a compound may also be called  less than  less than thermochromic greater than  greater than  which has the property of absorbing and/or reflecting different types of rays according to temperature due to a change in electronic structure. Vanadium dioxide has thus been studied for several years which has a structural transition at a temperature Tt=341 K or 68xc2x0 C.: below Tt the crystalline structure is monoclinic, whereas above T, the structure is rutile. This transition is associated with a sudden change in the electronic properties: the compound thus passes into the insulating state when the temperature is lower than Tt and into the metallic state when the temperature is greater than Tt; optically, this change manifests itself as deep modifications of the near and far infra-red absorbance and reflection properties.
In the rest of the description, the designation  less than  less than vanadium dioxide greater than  greater than  shall comprise vanadium dioxide commonly named VO2 or V2O4.
Various studies have recently been carried out on this compound, such as those that may be picked out in the publications S. M. Babulanam, Mat. Opt. Sol. Light Techn. 692 (1986) 8 and J. C. Valmalette, Sol. Energy Mater 33 (1994) 135. Studies have therefore been conducted on thin layers of vanadium dioxide deposited on various substrates: they have notably revealed the practical interest of the development of a material which is transparent to light but which only allows the infra-red part of the solar spectrum to pass through at low temperature. From this, the vanadium dioxide seems at the present time to be the only compound for which the transition is situated in a range of temperature and wavelengths suitable to the thermal regulation of the housing.
Moreover, this compound has the additional advantage of being able to undergo chemical substitutions with appropriate atoms such as defined further on and enabling a displacement of the temperature Tttowards lower temperatures.
Thus, many tests and researches have been developed to create thin layers of vanadium dioxide deposited on substrates, notably with the view to studying the optical transmittance in the visible and the near infra red; for this, various depositing techniques have been envisaged, such as cathodic spraying under vacuum, evaporation under beam, vapour phase chemical deposits and the  less than  less than sol-gel greater than  greater than  process.
According to the  less than  less than sol-gel greater than  greater than  process, vanadium dioxide is prepared from tetravalent vanadium by dissolution in a solvent, hydrolysis and condensation in order to gradually form a sol, then, by evaporating the solvent, forming a gel which is then submitted to a thermal treatment to give VO2, under a finely controlled atmosphere.
It is possible to directly form a VO2 film on a substrate, by soaking an appropriate substrate in the sol. The gel is thus formed directly on the substrate. Such a process of moistening or  less than  less than dip-coating greater than  greater than  is notably described in the U.S. Pat. No. 4,957,725.
It is however difficult to control the quality of the final film deposited, i. e. to place the complete piece or even its surface at high temperature in a uniform way and to control the interactions between the support and the gel thus deposited, etc . . . Thus, on the one hand, such methods which do not apply to the materials already constituted do not really enable an application on very large surfaces such as can be done with a surface coating composition such as paint and, on the other hand, the results obtained are neither repetitive nor reliable. Moreover, it is a very costly process when it comes to large surfaces.
Processes via the dry route generally exist which are very long (in the order of fifteen days) and are therefore very costly, which only enable obtaining molecules-grains in the order of 30 microns and more, which is not compatible with an incorporation into a paint without modifying the colour of it, which does not allow a homogeneous mixture and which does not bring about the property of optical transmission.
The problem posed is therefore one of being able to obtain a powder of low particle size which essentially comprises vanadium dioxide which is doped or not notably with tungsten which may notably be able to be incorporated in a liquid or viscous support with the view to obtaining a surface coating.
In a second aspect, the invention therefore relates to a method of obtaining microparticles of vanadium dioxide of formula V1xe2x88x92xMxO2 in which M is a doping metal and 0xe2x89xa6xxe2x89xa60.02, by pyrolysis of doped or non-doped ammonium hexavanadate, characterised in that said pyrolysis is carried out at a temperature between about 400xc2x0 C. and about 650xc2x0 C., with a temperature increase rate of at least 100xc2x0 C./min, and in that the gases resulting from said pyrolysis are kept in confinement and in direct contact with the reaction medium for a period of time of at least xc2xd hour, preferably 1 hour.
The use of ammonium hexavanadate (NH4)2V6O16 is known in industry for the manufacture of V2O5 commonly used as catalyst, but in which the tetravalent vanadium is considered as a non-catalytic impurity whose removal is sought. The tests which have been able to be done with this precursor in order to also obtain vanadium dioxide alone have not led to anything since V2O3 was obtained and every publication up to the present day maintains that it was not possible to obtain pure vanadium dioxide.